Many printed wiring boards in each of which a solder resist pattern that is a pattern-like solder resist film is formed on a substrate having a wiring line formed on the upper surface thereof have been used. The formation of fine solder resist patterns have been demanded in printed wiring boards with the decrease in size and the increase in density of electronic devices in which the printed wiring boards are installed.
As a method for forming a fine solder resist pattern, a method of applying a solder resist composition in an inkjet mode has been proposed. In an inkjet mode, the number of steps is reduced compared with a case in which a solder resist pattern is formed in a screen printing mode. Therefore, a solder resist pattern can be formed easily and efficiently in an inkjet mode.
When a solder resist composition is applied in an inkjet mode, it is required that the viscosity of the solder resist composition is low to some extent during the application of the composition. In recent years, on the other hand, an inkjet device that can print with a solder resist composition while warming the solder resist composition to 50° C. or higher. When the solder resist composition is warmed to 50° C. or higher in the inkjet device, the viscosity of the solder resist composition can become relatively low and, therefore, the ejection properties of the solder resist composition using the inkjet device would be further improved.
A solder resist composition that can be applied in an inkjet mode is disclosed in Patent Document 1 shown below. Patent Document 1 discloses a curable composition for inkjet which comprises a monomer having a (meth)acryloyl group and a heat-curable functional group, a photo-reactive diluent having a weight average molecular weight of 700 or less and a photopolymerization initiator. The viscosity of the curable composition for inkjet is 150 mPa·s or less at 25° C.